A common problem with vascular grafts is bleeding through holes punctured through the wall of a graft by suture needles or dialysis needles. These vascular grafts are most conventionally made of polyethylene terephthalate fabric or porous polytetrafluoroethylene tubing but materials of biologic origin such as human or bovine arteries or veins are also used. Suture needles used to create an anastomosis with these vascular grafts inevitably result in significant bleeding through the resulting holes that must be stopped prior to closure of the operative incision. Dialysis treatment of individuals suffering from renal failure requires that the blood of the individual be withdrawn, cycled through a dialysis machine and returned to the individual. A common approach to providing the necessary hemodialysis access is the use of an implanted arteriovenous vascular graft which may be subcutaneously pierced by a dialysis needle connected to the dialysis machine via a length of tubing.
Vascular grafts presently used for hemodialysis access can be improved in two regards. First, they typically must be implanted for about 14 days prior to puncture with a dialysis needle so that the graft has had time to become surrounded by fibrotic tissue and thereby reduce the risk of hemorrhage about the outer surface of the graft following removal of the dialysis needle. Second, these conventional grafts typically require the use of direct pressure to the puncture site for times in excess of three minutes in order to stop bleeding following removal of the needle. An arteriovenous graft that offered an improvement of either of these regards without compromising other positive characteristics would be a significant step forward in the field of hemodialysis access.
A proposed improvement in the field of arteriovenous access vascular grafts is described by U.S. Pat. No. 4,619,641 which teaches the construction of an access graft comprising two expanded polytetrafluoroethylene grafts in coaxial relationship with a space of about 1 mm disposed between the inner and outer grafts. The space is filled with a self-sealing elastomer such as silicone. While this construction does offer reduced bleeding after withdrawal of a dialysis needle, it is stiff and consequently difficult to work with during implantation.
Suture line bleeding resulting from graft penetration by a suture needle is frequently aggravated by tension applied to the sutures during construction of the anastomosis, the tension generally resulting in elongation and enlargement of the hole created by the penetration of the suture needle. Bleeding through suture holes must be stemmed before the access incision can be closed. Suture hole bleeding is thus responsible for both increased blood loss and increased time of operation. A vascular graft offering reduced suture bleeding would be of value in both regards.
U.S. Pat. No. 5,100,422 describes a blood vessel patch in the form of a sheet of porous polytetrafluoroethylene having an adhered exterior surface coating of an elastomer such as silicone or polyurethane wherein the coating is intended to reduce suture hole bleeding. The use of such coatings adversely affects the porosity of the patch material.
One embodiment of the present invention incorporates the use of fibers with conventional vascular graft constructions. Fibers have a long history of use in the construction of artificial vascular grafts. These devices were originally manufactured from polyethylene terephthalate (hereinafter PET) fibers woven or knitted into tubular forms. PET vascular grafts are rarely used for dialysis access. Further, PET grafts require pre-clotting with the patient's blood prior to implantation in order to avoid bleeding through the spaces between adjacent fibers.
U.S. Pat. No. 4,130,904 describes a double-walled vascular graft comprising two concentrically associated PET fabric tubes with a helical metal spring disposed between the inner and outer tubes. The intended purpose was to produce a crush resistant vascular graft without an exposed, external stiffening component.
Various patents have taught the use of electrostatic spinning techniques to manufacture vascular grafts from fibers. U.S. Pat. No. 4,323,525 teaches this technique as a method of forming a graft around a spinning mandrel or form that is removed after the tubular graft is formed. The fibers are formed from a solution or dispersion of a polymer that hardens and adheres to adjacent fibers after the forming of the graft, thereby creating a coherent tube.
U.S. Pat. No. 4,475,972 describes a vascular graft made by helically wrapping thermoplastic fibers in alternating directions about a mandrel and solvent bonding the contact points of overlying fibers. EP 0,407,692 describes a vascular graft comprising at least one tubular sleeve made according to U.S. Pat. No. 4,475,972 fitted coaxially over an inner vascular graft of biologic or synthetic material.
U.S. Pat. Nos. 4,632,842; 4,652,263; and 4,718,907 describe a method of providing longitudinal extensibility to otherwise substantially inextensible vascular graft tubes of woven PET fibers. The method comprises placing such a substantially inextensible tube of woven PET onto a close-fitting mandrel and compressing the tube longitudinally, thereby causing the longitudinally oriented warp fibers to be formed into loops that extend outwardly away from the surface of the mandrel. The compressed tube is then heat-set while still on the mandrel to provide a memory to the formed loops. After cooling and subsequent removal from the mandrel, the woven tube has a degree of longitudinal extensibility because the formed loops are extensible. This product is commercially available in the form of a tube woven from PET thread and provided with a lumenal coating of a fluorine-containing polymer applied by plasma polymerization techniques. Advertising literature for this product states that the fibers from which the tube is woven are punctured during cannulation and subsequently stretch back into place, apparently due to the loops formed into the warp threads.
U.S. Pat. No. 4,878,908 describes a vascular graft made from electrostatically spun fibers and having an inner blood contacting surface comprising fibers. The spun fiber tubular device may be either a self-supporting tube or may be a tubular liner for the interior of a second tubular structure. The fibers are of an organic material and are preferably synthetic materials such as PET, polytetrafluoroethylene (hereinafter PTFE) and silicone.
U.S. Pat. No. 5,061,276 describes a vascular graft comprising a composite tube of expanded PTFE and an elastomer, having an outer layer of elastomeric polymer fibers wound under tension about the circumference of the graft to cause retraction of the tubing from its original size. The wrapping of elastomeric fibers is provided with the intention of making the graft more compliant.
U.S. Pat. No. 5,116,360 teaches the construction of a composite vascular graft having an inner layer made of wound criss-crossing layers of fibers, an intermediate bonding layer of criss-crossing thermoplastic fibers of lower melt-point than the other construction materials, and an outer layer of porous mesh.